Laser pyrolysis method for producing carbon nano-spheres

ABSTRACT

The present invention encompasses methods and apparatus for creating metal nanoparticles embedded in a carbonaceous char, the conversion of an carbonaceous char with embedded metallic nanoparticles to graphite-encased nano-sized metal particles surrounded by char, the separation of the graphite encased metal particles from the char matrix, and the related preparation and isolation of carbon nanosphere materials with or without the enclosed metal nanoparticles, and the uses of such carbon nanospheres and graphite enclosed metal nanoparticles as supports and enhancers for fuel cell electrocatalysts and other applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of internationalapplication PCT/US04/04454 filed Feb. 17, 2004, which claims the benfitof the filing date of U.S. Provisional Application Ser. No. 60/448,924filed Feb. 22, 2003.

FIELD OF THE INVENTION

The present invention relates generally to the field of nanotechnologyand specifically to improved methods for the synthesis of carbonnanoparticles. This invention provides a novel method for the laserpyrolysis production of relatively homogeneous samples of hollow carbonnanospheres, and related intermediate products, derived via templatesynthesis, mediated by metal salts, from a carbonaceous substrate,preferably cellulose char, an inexpensive and renewable precursor. Suchnanospheres and related products have many potential uses, for exampleas blends in nanocomposites (e.g., in polymers and high temperatureglasses), as catalyst supports, and as nanoreaction chambers. Such newcarbon structures are particularly relevant to the optimization of fuelcell electrocatalyst supports.

BACKGROUND OF THE INVENTION

There is an immense interest in the fabrication of new carbon-basednanomaterials with highly curved graphitic structures. The interest inthese materials stems from their unique structural, mechanical andelectronic properties, and hence their potential for use in importantcommercial products. These materials, which include open and closednanotubes, carbon onions and graphitic nanocones, are mostlysynthesized, typically in low yield, via laser vaporization, resistiveheating or arc discharge methods, usually under high vacuum. See, forexample, Iijima, S. Nature, 1991, 354, 56; Ugarte, D. Nature, 1992, 359,707; and Krishnan, A.; Dujardin, E.; Treacy, M. M. J.; Hugdahl, J.;Lynum, S.; Ebbesen, T. W. Nature, 1997, 388, 451, which literaturereferences are incorporated herein by reference. Furthermore, theproducts of such conventional syntheses are often heterogeneous,typically being mixed with large amounts of undesirable materials andtherefore being difficult or impossible to purify. See, for example,Georgakilas, V.; Voulgaris, D.; Vázquez, E.; Prato, M.; Guldi, D. M.;Kukovecz, A.; Kuzmany, H. J. Am. Chem. Soc. 2002, 124, 14318 whichliterature reference is incorporated herein by reference. New andimproved methods for the fabrication of carbon nanoparticles would beespecially welcome if they could produce samples of both high purity andyield from readily available, renewable, inexpensive and benign startingmaterials.

Cellulose is unique among biopolymers in that, when it is charred below400° C. and above its decomposition temperature of 280° C., it producesan aromatic structure in which domains of polycyclic aromatichydrocarbon (PAH) anneal during such a charring step into largerensembles of five- and six-membered aromatic rings. See, for example,Herring, A. M.; McKinnon, J. T.; Petrick, D. E.; Gneshin, K. W.; Filley,J.; McCloskey, B. D. J. Annal. Appl. Pyrol. 2003, 66, 165, whichliterature reference is incorporated herein by reference. Otherbiopolymers, such as pectin, xylan and lignin, also produce charscontaining aromatic structure, but these other biopolymers do notexhibit this PAH annealing behavior on charring to the same extent asdoes cellulose. The extensive hydrogen bonding network between thedecomposing cellulose strands almost certainly plays an important rolein this behavior. The decomposition of cellulose has been studiedextensively, primarily for the purposes of understanding biomass energyprocesses, but cellulose has not previously been used for nanomaterialsynthesis.

Nanoparticles previously have been produced from aromatic and PAHmolecules and carbon soot, for example via catalyzed or templatedroutes. See, for example, Boese, R.; Matzanger, A. J.; Volhardt, K. P.C. J. Am. Chem. Soc. 1997, 119, 2052; Goel, A.; Hebgen, P.; VanderSande, J. B.; Howard, J. B. Carbon 2002, 40, 177; Hou, H.; Schaper, A.K.; Weller, F.; Greiner, A. Chem. Mater. 2002, 14, 3990; Hu, G.; Ma, D.;Cheng, M.; Liu, L.; Bao, X. Chem. Commun. 2001, 8630; and, Gherghel, L.;Kübel, C.; Lieser, G.; Räder, H.,-J.; Müllen, K. J. Am. Chem. Soc. 2002,124, 13130, which literature references are incorporated herein byreference. These methods are not well understood, but are stronglyinfluenced by the presence or absence of either a catalyst or a templatespecies. Similar structures, with a diameter of ca. 100 nm, have beenprepared by annealing carbon onions, produced by autoclave reaction ofNaCl and hexachloro benzene, at 1400° C. In these experiments, the NaClis intercalated in the graphitic layers of the carbon onions, and thevaporization of this salt results in the larger hollow carbonnanospheres.

Thus, carbon nanoparticles prepared in various ways and with manymorphological structures have existed prior to the current invention.The current technology in this field, however, is deficient orinadequate in one or more of the following ways:

-   -   1. Expensive processing operations to create the nanoparticle        products.    -   2. Expensive materials or catalysts required to create the        nanoparticle products.    -   3. Nanoparticle products are produced in low yields.    -   4. Nanoparticle products are produced in low purity.    -   5. Nanoparticle products are difficult or impossible to obtain        in pure form.    -   6. Nanoparticle products are not in the best morphological        configurations for use in the desired applications.

These and other deficiencies in or limitations of the prior art areovercome in whole or at least in part by the apparatus and methods ofthis invention.

OBJECTS OF THE INVENTION

A principal object of the present invention is to provide improvedmethods and apparatus for synthesizing carbon nanospheres from a charsubstrate material.

A more specific object of the present invention is to provide a methodto template cellulose char using nanoparticles of metal salts.

Another specific object of this invention is to produce and isolatemetal salt nanoparticles from the char using mild oxidation.

Still another object of this invention is to convert the cellulose charencased metal salt particles into graphite encased metal nanoparticles.

Yet another object of this invention is to isolate the graphite encasednanoparticles from the char matrix.

Another object of this invention is to isolate the graphite shells ofthe nanoparticles as hollow carbon nanospheres.

Still another object of this invention is to use either the graphiteencased metal nanoparticles or the hollow carbon nanospheres as supportsfor platinum or other precious metals or their alloys to form anelectrocatalyst component for use in the membrane electrode assembly ofa proton exchange membrane fuel cell.

These and other objects, advantages and benefits of this invention willbe better understood from the following description read in conjunctionwith FIGS. 1-7.

SUMMARY OF THE INVENTION

Methods for synthesizing hollow carbon nanospheres from a metalsalt-doped carbon-based substrate are provided wherein carbonnanospheres are prepared by pyrolyzing chars at high temperatures. Thechars are doped with selected metals/metal salts which serve astemplates for creating the nanospheres. The pyrolysis heating may takeplace using a laser, other intense light sources, or other energysources capable of heating a solid to temperatures in excess of 2000 K.The size of the carbon nanospheres according to this invention may beoptimized by adjusting the amount and type of metal catalyst used, thetemperature, pressure, temperature ramp rate, and other conditions usedto create the char; and the temperature, pressure, temperature ramprate, and other conditions used to pyrolyze the char. The methods ofthis invention may also be used to create metal nanoparticles, which canbe isolated from the carbonaceous material, and carbon nanospheresfilled with metal, each of which has independent utility in variousapplications. The carbon nanospheres of this invention have variousapplications such as in fuel cell electrode supports, nanoreactionchambers, blending agents for polymers, and strengthening agents forhigh temperature glasses.

More specifically, the present invention provides improved methods andapparatus for synthesizing carbon nanospheres from a charable,carbonaceous substrate material. The char may be prepared from celluloseor other carbon substrate materials. In a preferred embodiment, metalspreferably in the form of metal salts are added to the uncharredsubstrate in amounts ranging from about 1-99% by weight, more preferablyabout 10-50% by weight, most preferably about 15-35% by weight. Thesemetals/salts are believed to help form nanometer-scale particles in thechar. These nanometer-scale particles then serve as templates forcreating the desired carbon nanospheres. Preferred metal salts for usein this invention are the salts of first row transition metals, e.g.,the 3d transition metals, and particularly, the salts of Ti, V, Cr, Mn,Fe, Co, Ni and Cu. A specific example is nickel chloride. Thenanospheres are formed by heating the specially prepared char substrateto very high pyrolyzing temperatures, on the order of about 2000 K orgreater. In one preferred embodiment, the heat source for heating thechar substrate is an infrared laser, but other heat sources may also beused effectively in practicing this invention.

After being formed during the high temperature pyrolysis step describedabove, the carbon nanospheres can be removed from the amorphous carbonthat surrounds them, for example by digesting the amorphous material inconcentrated nitric acid. The nanosphere structures formed in accordancewith this invention are substantially if not totally resistant to attackby the acid. Transmission electron microscope (TEM) analysis ofnanospheres formed in accordance with this invention shows that thesamples are almost totally homogenous. That is, they are uniform in size(for example, about 30-40 nm in diameter) with very little or no foreignmaterial. The present invention provides a means of manufacturing largequantities of such nanospheres having superior purity, homogeneity andother properties for relatively low cost compared with prior arttechniques.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a transmission electron microscope (TEM) image of a portionof a charred mixed nickel chloride/cellulose pellet wherein nickelchloride nanoparticles are surrounded by amorphous carbon.

FIG. 2 shows the charred mixed nickel chloride/cellulose pellet of FIG.1 after it has been subjected to a laser pyrolysis treatment inaccordance with a preferred embodiment of this invention, illustratingthat the nickel nanoparticles are now encased in graphite and aresurrounded by amorphous carbon.

FIG. 3 shows hollow graphitic nanospheres formed in accordance with thisinvention.

FIG. 4 a shows an X-ray diffraction (XRD) image of a charred nickelchloride/cellulose pellet.

FIG. 4 b shows an XRD image of a charred nickel chloride/cellulosepellet after a laser pyrolysis treatment in accordance with a preferredembodiment of this invention.

FIG. 4 c shows an XRD image of a product sample prepared in accordancewith this invention after a digestion/separation step, the productsample being composed primarily of hollow carbon nanospheres with somenickel nanoparticles and a small fragment of an unidentified phase.

FIG. 5 shows a diffuse reflectance infrared spectrum of hollow carbonnanospheres formed in accordance with this invention showing somesurface functionalization.

FIG. 6 shows platinum nanoparticles attached to hollow carbonnanospheres formed in accordance with this invention.

FIG. 7 illustrates the preparation of carbon nanospheres in accordancewith this invention from nickel chloride doped cellulose.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is based on the novel discovery that the chemistryof charring carbonaceous substrate materials, particularly cellulose,can be modified in various surprising and advantageous ways by theaddition of certain particular types of metals, e.g., in the form ofmetal salts, which can act as a template for the charring material.Furthermore, these metal particles embedded in the char have also beenfound to favorably influence the subsequent pyrolysis chemistry andallow the production of the desired carbon nanoparticle morphology.Finally, the intermediate production and isolation of the nano-sizedmetal templates (metal nanoparticles) in accordance with this inventionis a further desired outcome.

Transition metals have been known previously for their efficientproduction of single- and multi-wall nanotubes. See, for example,Colomer, J. F.; Bister, G.; Willems, I.; Kónya; Fonseca, A.; VanTendeloo, G.; Nagy, J. B. Chem Commun. 1999, 1343, which literaturereference is incorporated herein by reference. In particular, the entireseries of 3d transition metals, such as nickel, cobalt and iron, hasbeen found to be particularly effective for their efficient productionof single- and multi-wall nanotubes. In the following description ofnanosphere formation in accordance with the techniques of thisinvention, nickel was selected as a preferred transition metal, but itwill be understood that other similar metals can also be used withsomewhat different results. The initial charring process (T<400° C.)produces amorphous carbon. This carbon becomes graphitic during thesubsequent pyrolysis step of the invention (T>2000° C.). In a preferredembodiment, a laser is used as the heat source to rapidly pyrolyze thecellulose char and produce graphitic nanostructures. For example, carbondioxide lasers can be operated in continuous wave mode to produce carbonnanotubes from graphite/bi-metallic targets in accordance with thisinvention. See, for example, Maser, W. K.; Munoz, E.; Beito, A. M.;Martinez, M. T.; de la Fuente, G. F.; Naniette, Y.; Anglaret, E.;Sauvajor, J., l. Chem. Phys. Lett. 1998, 292, 587, which literaturereference is incorporated herein by reference, for a general descriptionof carbon dioxide lasers. This method was adapted for purposes of thisinvention to enable easy scale up of the process using a continuouslyfed char stream of a suitable, previously prepared metal-doped charmaterial in accordance with this invention.

Illustrative Char Production

The following description describes one exemplary technique forpreparing a metal-doped char in accordance with this invention. Toproduce the char material, powdered cellulose (4 grams, brand nameAvicel) was ground with nickel (II) chloride hexahydrate (1 gram) usingan agate mortar and pestle until a fine powder was formed. TheNiCl₂.6H₂O was then dried in an oven at about 80° C. for at least oneday before it was used for any of the subsequent experiments. Theinitial size of the powder particles was approximately 30 μm. Once theNiCl₂/Avicel mixture was prepared, it was allowed to sit in an 80° C.oven for at least one hour. The powdered mixture was then pressed in a2.5 cm die at about 10,000 psi pressure for 30 seconds. The resultingsolid pellet was then charred in a tube furnace at 375° C. for about 30minutes using a 5-minute heat ramp procedure starting from ambienttemperature until reaching 375° C. Charring occurred under an inertatmosphere (argon). The inert gas flowed through the 31 mm diametercharring tube at 13.3 cm³/s. During the charring process, approximately80% of the pellet mass was lost to the flowing gas as volatilecompounds. After charring, the nickel in the pellet had formedcrystallites of 10-20 nm in size surrounded by an amorphous carbonaceousmaterial, as shown in the image of FIG. 1.

After charring, the pellet was allowed to cool in a glove bag undernitrogen for approximately 5-10 minutes before being mounted onto aspindle for the subsequent pyrolysis operation.

If metal nanoparticles are a desired end product, at least a firstportion of the char material can be separated from the remaining secondportion for this purpose. The metal nanoparticles may be separated fromthe carbonaceous material by treating the first portion with a mildoxidation in air. The DRIFTS of this char is very similar to that ofundoped cellulose charred under identical conditions, with twoexceptions. First, the ν(C═O) band, 1709 cm⁻¹, is reduced; and, second,anew sharp aromatic γ(CH) band appears at 764 cm⁻¹ indicating that thearomatic ring structure is less substituted than in pure cellulose char.As expected, the XRD imaging pattern, shown in FIG. 4 a, isrepresentative of a mixture of NiCl₂ and amorphous carbon.

Illustrative Pyrolysis Process

At least a portion, e.g., the second portion, of the charred materialproduced according to the preceding illustrative char production processwas laser pyrolyzed in a pyrolysis chamber using a carbon dioxide laserwith a power of 58.5 W. The temperatures achieved by this heatingprocess were in excess of the temperature of sublimation of anhydrousNiCl₂, i.e., 973° C. The pyrolysis chamber was evacuated and filled withargon twice before the pellet was laser pyrolyzed. At a pressure of 0.5torr, the edge of the pellet was irradiated by a 20 W/mm² CO₂ laser beamwhile the pellet was spun by a stepper motor continuously at an angularvelocity of 1.2 rev/min (which corresponds to a linear velocity at theedge of the pellet of about 1.63 mm/s). The pellet was allowed tocomplete one full revolution in the laser irradiation. Once thepyrolysis was completed, the pellet was taken out of thepyrolysis/combustion chamber and allowed to sit overnight in ambientconditions.

The major products from this reaction at lower laser powers of about 6.5W, as detected by molecular beam mass spectroscopy, are HCl, CO and CO₂.The NiCl₂ is believed to be involved in an oxidation reaction with theamorphous oxygenated PAH producing carbon, Ni(0) and HCl; and, the COand CO₂ are simply byproducts of pyrolysis of the char. At low laserpowers of about 6.5 to about 50 W, this grey material was found toconsist of amorphous carbon containing intercalated nickel. When thepyrolysis was carried out at a high laser power, e.g., 58.5 W, the solidmaterial showed new peaks in the XRD imaging patterns, which seem torepresent nickel metal and graphitic carbon as seen in FIG. 4 b.Examination of the material by TEM imaging reveals 30 nm particles ofpure Ni, each surrounded by a 5-10 nm shell of graphite as shown in FIG.2. These Ni-graphite particles are surrounded by amorphous carbon.

In order to remove the amorphous carbon and isolate the nano-sized metalparticles in accordance with this invention, the resultant material issubjected to a further purification treatment, for example by an aciddigestion with refluxing concentrated nitric acid. To do this, thepellet was ground to as fine a powder as possible using a mortar andpestle. The ground pellet was placed in 20 ml DI water and sonicated forabout 10 minutes in order to form a very fine powder. The ground pelletwas then centrifuged and dried before it was placed in nitric acid.Digestion was completed while refluxing in concentrated nitric acid for4 hours with continuous stirring using a stir bar. Once the 4 hourperiod was over, 20 ml of DI water was added to the nitric acidsolution. To separate the desired metal particle end product from thenitric acid, the solution was filtered using a glass frit filter andwashed with DI water until the effluent from the filter was no longeracidic (as indicated by litmus paper testing). The process yielded 100mg of a black powder on drying.

The SRD image of the black powder, as shown in FIG. 4 c, was dominatedby peaks assigned to graphitic carbon, some residual nickel metal, and asmall amount of an as yet unidentified phase. Line width analysis of theXRD image using the Sherrer equation resulted in crystalline sizes forthe graphite of 5.2 in and 29.4 nm for the nickel. Individual TEMimaging of multiple (more than 8) samples of this materials have allrevealed extremely homogeneous samples of clumps of 40-50 nm diameterhollow carbon nanospheres, as shown in FIG. 3. The shells of thenanospheres have been determined to be composed of highly ordered layersof up to 60 concentric curbed graphitic sheets. The spacing of thegraphite layers, 3.41 Å, correlates with a temperature of formation of2250° C. See, for example, Mantel, C. Carbon and Graphite Handbook,Wiley and Sons, New York, 1968, which literature reference isincorporated herein by reference. The TEM imaging showed carbonnanospheres filled with nickel metal comprising approximately 1% of thesample. The DRIFTS of this material revealed weak features assigned tosurface carbonyl and hydroxyl, 1764 and 3181 cm⁻¹, respectively.

Control experiments using pure Avicel cellulose but with no metal salts,charred in an identical way to the metal-doped material as describedabove, produced no insoluble material after nitric acid digestion.Clearly the nickel doping has affected the charring chemistry in someimportant way, it is believed by disrupting the hydrogen bond network ofthe char and templating the oxygenated-PAH domains, as seen in FIG. 7.The size of the carbon nanospheres thus produced depends on the size ofthe precursor NiCl₂ crystallites in the char. The size of the NiCl₂crystallites can, in turn, be varied by changing the charring conditionsalso in accordance with this invention. Alternatively, different metalsalts can be used to dope the pre-char carbonaceous material to alsovary the size of the carbon nanospheres produced. These carbonnanospheres have been determined to be electrically conductive as wellas to have functionalized peripheries.

Applications of Carbon Nanospheres

Because this invention results in the formation of novel and newlydeveloped materials, the full scope of applications for the uniquecarbon nanospheres produced by the methods of this invention is not yetfully appreciated. Despite this uncertainty, it is believed thatnanospheres produced in accordance with this invention will prove to beextremely useful in at least three technology areas: fuel cell electrodecatalyst supports; high-temperature glass additives; and polymerblending agents.

Fuel Cell Electrocatalyst Supports:

The electrocatalyst support of a PEM fuel cell must perform fourseparate operations. First, it must provide a means of efficientlydispersing the expensive platinum (or other precious metal)electrocatalyst, i.e., disperse the platinum in the smallestcrystallites possible in order to maximize the effective surface area.Second, it must provide continuous bulk transport pathways for the fuelor oxidant to the electrocatalyst. Third, it must be electricallyconductive to allow transport of the electrons. Finally, theelectrocatalyst support must allow proton transport to the membrane. Theproperties of carbon nanospheres prepared in accordance with thisinvention allow the first three electrocatalyst support operations to bedesirably carried out without additives. The simple addition of a protonconductor, such as Nafion, to an electrocatalyst support based on carbonnanospheres prepared in accordance with this invention thereby allows anearly ideal electrocatalyst support to be formed. To create thesematerials, the surface of the nanosphere product was modified by a mildoxidation using 4M HNO₃ to convert the anhydride surface functionalitiesto carboxylic acid groups. The resultant black powder was refluxed withcholorplatinic acid in ethylene glycol for 6 hours followed by washingand drying in air. The TEM image of this electrocatalyst material isshown in FIG. 6.

High Temperature Stable Glasses:

Carbide and nitride based ceramic glasses (e.g., SiCN, SiOC, SiOCN) haveshown the greatest potential for structural applications in hightemperatures and harsh environments. Unfortunately, many of the carbidesand nitrides processed through traditional powder consolidation andsintering are unstable in oxygen-containing environments due to passiveand active oxidation. Polymer-derived carbide and nitride glasses, onthe other hand, have shown excellent resistance to oxidation attemperatures up to 1450° C. Although the exact mechanism is not yet wellunderstood, it is believed that free carbon incorporated into the glassnetwork structure is inherently less prone to oxidation and corrosionthan polycrystalline ceramics. The addition of hollow carbon nanospheresprepared in accordance with this invention provides a promising methodto further enhance the properties of these glass compositions byintroducing additional free carbon into the O-, N-based glasses in aneffort to prevent crystallization at temperatures up to 1700° C. andyield a lightweight, creep-resistant, HT-stable glass.

Totally Renewable Nanocomposite Plastics:

The field of polymers produced from corn-based polylactic acid (PLA) isa rapidly exploding area both in terms of research and commercialinterest. However, the current PLA materials have morphological andperformance problems that must be addressed before they make widespreadmarket penetration. Blending PLA plastics with carbon nanospheresprepared in accordance with this invention shows promise in bothaddressing the plastic performance issues and producing a material basedentirely on renewable resources.

These and other important applications for the carbon nanosphereproducts and the nano-sized metal particles produced in accordance withthis invention will be apparent to those skilled in the art, and allsuch uses and applications of the novel products of this invention areintended to be covered by the appended claims.

It will also be apparent to those skilled in the art that other changesand modifications may be made in the above-described apparatus andmethods for pyrolysis of metal-doped carbonaceous chars for producingcarbon nanospheres without departing from the scope of the inventionherein, and it is intended that all matter contained in the abovedescription shall be interpreted in an illustrative and not a limitingsense.

1. A method for synthesizing hollow carbon nanospheres andnanometer-scale metallic particles from a carbon-based substrate dopedwith a metal salt, and for recovering either one or both of theseproducts, comprising the sequential steps of: (a) doping a carbon-basedsubstrate with about 1-99 wt. % of a metal salt; (b) heating the dopedsubstrate in an inert gas atmosphere to a first heating temperature ofabout 200° C. to about 400° C. for a period sufficient to produceinitial charring of the substrate to form amorphous carbon and metalnanoparticles; (c) cooling the charred substrate; (d) optionallyseparating a first portion of the charred substrate from the remainingcharred substrate and recovering metal nanoparticles if desired fromsaid first portion of the charred substrate; and, (e) pyrolyzing atleast a portion of the remaining charred substrate by heating it to asecond heating temperature of 2000° C. or higher in an inert gasatmosphere for a period sufficient to produce graphitic nanostructureshaving the properties of being almost totally homogeneous andsubstantially resistant to attack by acid.
 2. A method according toclaim 1 wherein the metal salt is selected from the group consisting ofthe salts of transition metals.
 3. A method according to claim 1 whereinthe metal salt is selected from the group consisting of the salts of Ti,V, Cr, Mn, Fe, Co, Ni and Cu.
 4. A method according to claim 1 furtherincluding the step of blending the carbon nanospheres with ahigh-performance plastic composition consisting essentially ofpolylactic acid (PLA).
 5. A method according to claim 1 wherein thecarbon-based substrate is doped with about 10-50 wt. % of a metal salt.6. A method according to claim 1 wherein the metal salt is mixed withthe carbon-based substrate in a weight ratio of about 1 part by weightof metal salt to about 4 parts by weight of carbon-based substrate.
 7. Amethod according to claim 1 wherein step (e) is carried out using alaser at a laser power above about 50 W.
 8. A method according to claim1 wherein the carbon-based substrate is doped with about 15-35 wt. % ofa metal salt.
 9. A method according to claim 1 wherein substantially allof the charred substrate is subjected to the pyrolysis treatment of step(e).
 10. A method according to claim 1 further including the step ofdispersing a precious metal along surfaces of the carbon nanospheres.11. A method according to claim 10 further wherein the precious metal isplatinum.
 12. A method according to claim 1 further including the stepof blending the carbon nanospheres with a carbide and/or nitride-basedceramic material.
 13. A method according to claim 1 further includingthe step of blending the carbon nanospheres with a high-performanceplastic composition consisting essentially of polylactic acid (PLA). 14.A method for synthesizing hollow carbon nanospheres and nanometer-scalemetallic particles from a carbon-based substrate doped with a metalsalt, and for recovering either one or both of these products,comprising the sequential steps of: (a) doping a carbon-based substratewith about 1-99 wt. % of a metal salt; (b) heating the doped substratein an inert gas atmosphere to a first heating temperature of about 200°C. to about 400° C. for a period sufficient to produce initial charringof the substrate to form amorphous carbon and metal nanoparticles; (c)cooling the charred substrate; (d) optionally separating a first portionof the charred substrate from the remaining charred substrate andrecovering metal nanoparticles if desired from said first portion of thecharred substrate; and, (e) pyrolyzing at least a portion of theremaining charred substrate by heating it to a second heatingtemperature of 2000° C. or higher in an inert gas atmosphere for aperiod sufficient to produce graphitic nanostructures having theproperties of being almost totally homogeneous and substantiallyresistant to attack by acid; wherein the metal salt is nickel (II)chloride hexahydrate (NiCl₂.6H₂O).
 15. A method according to claim 14further including the step of dispersing a precious metal along surfacesof the carbon nanospheres.
 16. A method according to claim 14 furtherincluding the step of blending the carbon nanospheres with a carbideand/or nitride-based ceramic material.
 17. A method according to claim14 further including the step of blending the carbon nanospheres with ahigh-performance plastic composition consisting essentially ofpolylactic acid (PLA).
 18. A method for synthesizing hollow carbonnanospheres and nanometer-scale metallic particles from a carbon-basedsubstrate doped with a metal salt, and for recovering either one or bothof these products, comprising the sequential steps of: (a) doping acarbon-based substrate with about 1-99 wt. % of a metal salt; (b)heating the doped substrate in an inert gas atmosphere to a firstheating temperature of about 200° C. to about 400° C. for a periodsufficient to produce initial charring of the substrate to formamorphous carbon and metal nanoparticles; (c) cooling the charredsubstrate; (d) optionally separating a first portion of the charredsubstrate from the remaining charred substrate and recovering metalnanoparticles if desired from said first portion of the charredsubstrate; and, (e) pyrolyzing at least a portion of the remainingcharred substrate by heating it to a second heating temperature of 2000°C. or higher in an inert gas atmosphere for a period sufficient toproduce graphitic nanostructures having the properties of being almosttotally homogeneous and substantially resistant to attack by acid;wherein the carbon-based substrate is cellulose.
 19. A method forsynthesizing hollow carbon nanospheres and nanometer-scale metallicparticles from a carbon-based substrate doped with a metal salt, and forrecovering either one or both of these products, comprising thesequential steps of: (a) doping a carbon-based substrate with about 1-99wt. % of a metal salt; (b) heating the doped substrate in an inert gasatmosphere to a first heating temperature of about 200° C. to about 400°C. for a period sufficient to produce initial charring of the substrateto form amorphous carbon and metal nanoparticles; (c) cooling thecharred substrate; (d) separating a first portion of the charredsubstrate from the remaining charred substrate and recovering metalnanoparticles if desired from said first portion of the charredsubstrate; and, (e) pyrolyzing at least a portion of the remainingcharred substrate by heating it to a second heating temperature of 2000°C. or higher in an inert gas atmosphere for a period sufficient toproduce graphitic nanostructures having the properties of being almosttotally homogeneous and substantially resistant to attack by acid;wherein at least a first portion of the charred substrate is cooled andsubjected to a mild oxidation to separate metal nanoparticles from thecarbonaceous material.
 20. A method according to claim 19 wherein themetal salt is selected from the group consisting of the salts oftransition metals.
 21. A method according to claim 19 further includingthe step of dispersing a precious metal along surfaces of the carbonnanospheres.
 22. A method according to claim 19 further including thestep of blending the carbon nanospheres with a carbide and/ornitride-based ceramic material.
 23. A method according to claim 19further including the step of blending the carbon nanospheres with ahigh-performance plastic composition consisting essentially ofpolylactic acid (PLA).
 24. A method for synthesizing hollow carbonnanospheres and nanometer-scale metallic particles from a carbon-basedsubstrate doped with a metal salt, and for recovering either one or bothof these products, comprising the sequential steps of: (a) doping acarbon-based substrate with about 1-99 wt. % of a metal salt; (b)heating the doped substrate in an inert gas atmosphere to a firstheating temperature of about 200° C. to about 400° C. for a periodsufficient to produce initial charring of the substrate to formamorphous carbon and metal nanoparticles; (c) cooling the charredsubstrate; (d) optionally separating a first portion of the charredsubstrate from the remaining charred substrate and recovering metalnanoparticles if desired from said first portion of the charredsubstrate; and, (e) pyrolyzing at least a portion of the remainingcharred substrate by heating it to a second heating temperature of 2000°C. or higher in an inert gas atmosphere for a period sufficient toproduce graphitic nanostructures having the properties of being almosttotally homogeneous and substantially resistant to attack by acid;wherein the pyrolysis treatment utilizes a laser as the heat source torapidly pyrolyze the metal-doped charred substrate.
 25. A methodaccording to claim 24 wherein the metal salt is selected from the groupconsisting of the salts of transition metals.
 26. A method according toclaim 24 further including the step of blending the carbon nanosphereswith a carbide and/or nitride-based ceramic material.
 27. A method forsynthesizing hollow carbon nanospheres and nanometer-scale metallicparticles from a carbon-based substrate doped with a metal salt, and forrecovering either one or both of these products, comprising thesequential steps of: (a) doping a carbon-based substrate with about 1-99wt. % of a metal salt; (b) heating the doped substrate in an inert gasatmosphere to a first heating temperature of about 200° C. to about 400°C. for a period sufficient to produce initial charring of the substrateto form amorphous carbon and metal nanoparticles; (c) cooling thecharred substrate; (d) optionally separating a first portion of thecharred substrate from the remaining charred substrate and recoveringmetal nanoparticles if desired from said first portion of the charredsubstrate; and, (e) pyrolyzing at least a portion of the remainingcharred substrate by heating it to a second heating temperature of 2000°C. or higher in an inert gas atmosphere for a period sufficient toproduce graphitic nanostructures having the properties of being almosttotally homogeneous and substantially resistant to attack by acid;wherein step (e) is carried out using a carbon dioxide laser.
 28. Amethod for synthesizing hollow carbon nanospheres and nanometer-scalemetallic particles from a carbon-based substrate doped with a metalsalt, and for recovering either one or both of these products,comprising the sequential steps of: (a) doping a carbon-based substratewith about 1-99 wt. % of a metal salt; (b) heating the doped substratein an inert gas atmosphere to a first heating temperature of about 200°C. to about 400° C. for a period sufficient to produce initial charringof the substrate to form amorphous carbon and metal nanoparticles; (c)cooling the charred substrate; (d) optionally separating a first portionof the charred substrate from the remaining charred substrate andrecovering metal nanoparticles if desired from said first portion of thecharred substrate; and, (e) pyrolyzing at least a portion of theremaining charred substrate by heating it to a second heatingtemperature of 2000° C. or higher in an inert gas atmosphere for aperiod sufficient to produce graphitic nanostructures having theproperties of being almost totally homogeneous and substantiallyresistant to attack by acid; wherein step (e) is carried out using aninfrared laser.
 29. A method for synthesizing hollow carbon nanospheresand nanometer-scale metallic particles from a carbon-based substratedoped with a metal salt, and for recovering either one or both of theseproducts, comprising the sequential steps of: (a) doping a carbon-basedsubstrate with about 1-99 wt. % of a metal salt; (b) heating the dopedsubstrate in an inert gas atmosphere to a first heating temperature ofabout 200° C. to about 400° C. for a period sufficient to produceinitial charring of the substrate to form amorphous carbon and metalnanoparticles; (c) cooling the charred substrate; (d) optionallyseparating a first portion of the charred substrate from the remainingcharred substrate and recovering metal nanoparticles if desired fromsaid first portion of the charred substrate; and, (e) pyrolyzing atleast a portion of the remaining charred substrate by heating it to asecond heating temperature of 2000° C. or higher in an inert gasatmosphere for a period sufficient to produce graphitic nanostructureshaving the properties of being almost totally homogeneous andsubstantially resistant to attack by acid; wherein step (e) is carriedout using a plasma torch.
 30. A method according to claim 29 wherein themetal salt is selected from the group consisting of the salts oftransition metals.
 31. A method according to claim 29 further includingthe step of blending the carbon nanospheres with a carbide and/ornitride-based ceramic material.
 32. A method according to claim 29further including the step of blending the carbon nanospheres with ahigh-performance plastic composition consisting essentially ofpolylactic acid (PLA).
 33. A method for synthesizing hollow carbonnanospheres and nanometer-scale metallic particles from a carbon-basedsubstrate doped with a metal salt, and for recovering either one or bothof these products, comprising the sequential steps of: (a) doping acarbon-based substrate with about 1-99 wt. % of a metal salt; (b)heating the doped substrate in an inert gas atmosphere to a firstheating temperature of about 200° C. to about 400° C. for a periodsufficient to produce initial charring of the substrate to formamorphous carbon and metal nanoparticles; (c) cooling the charredsubstrate; (d) optionally separating a first portion of the charredsubstrate from the remaining charred substrate and recovering metalnanoparticles if desired from said first portion of the charredsubstrate; and, (e) pyrolyzing at least a portion of the remainingcharred substrate by heating it to a second heating temperature of 2000°C. or higher in an inert gas atmosphere for a period sufficient toproduce graphitic nanostructures having the properties of being almosttotally homogeneous and substantially resistant to attack by acid;further comprising the steps of repeating steps (a) to (e) while varyingthe amount and type of metal dopant in the carbon-based substrate; thetemperature, pressure, and temperature ramp-up rate in step (b); and thetemperature, pressure and heating conditions in step (e), in order toform carbon nanospheres of different sizes and properties for particularapplications.
 34. A method for synthesizing hollow carbon nanospheresand nanometer-scale metallic particles from a carbon-based substratedoped with a metal salt, and for recovering either one or both of theseproducts, comprising the sequential steps of: (a) doping a carbon-basedsubstrate with about 1-99 wt. % of a metal salt; (b) heating the dopedsubstrate in an inert gas atmosphere to a first heating temperature ofabout 200° C. to about 400° C. for a period sufficient to produceinitial charring of the substrate to form amorphous carbon and metalnanoparticles; (c) cooling the charred substrate; (d) optionallyseparating a first portion of the charred substrate from the remainingcharred substrate and recovering metal nanoparticles if desired fromsaid first portion of the charred substrate; and, (e) pyrolyzing atleast a portion of the remaining charred substrate by heating it to asecond heating temperature of 2000° C. or higher in an inert gasatmosphere for a period sufficient to produce graphitic nanostructureshaving the properties of being almost totally homogeneous andsubstantially resistant to attack by acid; further comprising the stepof purifying the carbon nanospheres produced in step (e), said step ofpurifying comprising an acid digestion step or a mild gas-phaseoxidation step.
 35. A method according to claim 34 wherein said step ofpurifying the carbon nanospheres comprises an acid digestion usingconcentrated nitric acid to digest amorphous material in the reactionproduct.